In a typical radio communications network, wireless terminals, also known as mobile stations and/or user equipments (UEs), communicate via a Radio Access Network (RAN) to one or more core networks. The RAN covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” in Universal Mobile Telecommunications System (UMTS) or “eNodeB” Long Term Evolution (LTE). A cell is a geographical area where radio coverage is provided by the radio base station at a base station site or an antenna site in case the antenna and the radio base station are not collocated. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. Another identity identifying the cell uniquely in the whole mobile network is also broadcasted in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipments within range of the base stations.
In some versions of the RAN, several base stations are typically connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller (RNC) or a base station controller (BSC), which supervises and coordinates various activities of the plural base stations connected thereto. The RNCs are typically connected to one or more core networks.
A UMTS is a third generation mobile communication system, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). The UMTS Terrestrial Radio Access Network (UTRAN) is essentially a RAN using Wideband Code Division Multiple Access (WCDMA) and/or High Speed Packet Access (HSPA) for user equipments. In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for e.g. third generation networks and further generations, and investigate enhanced data rate and radio capacity.
At 3GPP RAN#54 plenary meeting a Work Item (WI) on Multiple Input Multiple Output (MIMO) with 64 Quadrature amplitude modulation (QAM) for High Speed Uplink Packet Access (HSUPA) was initiated, see RP-111642, “MIMO with 64 QAM for HSUPA”. The WI initialization was a result of extensive studies regarding potential benefits and solutions performed during the study item (SI) phase; see TR 25.871, “Uplink Multiple Input Multiple Output (MIMO) for High Speed Packet Access (HSPA)” v11.0.0. for a summary of the findings.
For a UE configured in uplink MIMO mode either single or dual-stream transmissions can take place; also referred to as rank1 and rank2 transmissions, respectively. Dual-stream transmissions are mainly useful in environments with high Signal-to-interference-and-noise ratios (SINR), and where the channel exhibits scattering, and/or cross-polarized antennas are used. When single-stream transmissions are scheduled, the pre-coding gain will instead improve the link budget.
Grant and E-TFC Selection Procedure
For legacy UEs, Transport Block Size (TBS) is set and controlled via a grant & Enhanced Dedicated Transport Channel Transport Format Combination (E-TFC) selection procedure. Enhanced Dedicated Transport Channel may be abbreviated as E-DCH. The grant, which is defined as the power ratio between the Enhanced Dedicated Transport Channel Dedicated Physical Data Channel (E-DPDCH) and Dedicated Physical Control Channel (DPCCH), is controlled by the network, for example the RNC or the radio base station, via absolute and relative grants transmitted over an E-DCH Absolute Grant Channel (E-AGCH) and E-DCH Relative Grant Channel (E-RGCH) channels, respectively. The grant is essentially a power measure which allows the network to control the interference a certain UE is allowed to create. The grant gives the UE an “upper bound” on how much data it may transmit, and the exact number of data bits will depend on power and buffer statuses, and essentially a formula configured via a number of reference values signaled by higher-layers; see e.g. sub-clause 11.8.1.4 in TS-25.321, “Medium Access Control (MAC) protocol specification”, v11.0.0. However, the grant may sometimes generate a non-optimal transmission for UEs in MIMO operation resulting in a reduced performance of the wireless communications network.